This invention relates in general to techniques for displaying information and, more particularly, to a method and apparatus for displaying information to a person who is operating some type of equipment or device, such as the driver of a vehicle.
Over the last several years, there has been an increase in the sale of automobiles having a head-up display (HUD) which is mounted in the dashboard, and which projects a visible image onto the inside surface of the windshield. The vast majority of these HUDs are used for night vision systems, in which an infrared camera unit is mounted at the front of the vehicle, for example in the center of the grille, and in which the HUD projects images from the camera unit onto the inside surface of the windshield during operation of the vehicle at night.
In this regard, the camera unit is capable of detecting thermal energy which originates at locations well beyond the range of the headlights. Thus, for example, if a person or a deer wanders into the road far ahead of the vehicle, the infrared camera unit will detect thermal energy from the person or deer well before the person or deer is illuminated by the headlights of the vehicle, and the HUD will produce a very visible indication to the driver that there is something in the road ahead. This facilitates safer driving at night.
The presentation of this information by means of a HUD makes this information more readily visible to the driver, who can easily see this information without taking his or her eyes off the road ahead. In contrast, if the information was displayed in the standard instrument cluster in the dashboard, rather than with a HUD, the driver would have to periodically take his or her eyes off the road and glance down at the instrument cluster in order to monitor this information.
It would be desirable to also use this type of HUD during the day. Of course, infrared images of the type displayed in the night mode would be of reduced value and interest in the day mode, but other information could be displayed, including operational information about the vehicle such as its current speed, Global Positioning Satellite (GPS) information such as the current location of the vehicle, and/or any other telematics that the driver might want to see. However, it has not previously been practical to provide such a system which has a size and cost suitable for the high-volume consumer market for automobiles and other vehicles.
In this regard, one consideration is that existing HUDs include a relatively large projection mirror with a high degree of reflectivity, in order to facilitate proper operation at night. During the day, however, this large and highly reflective mirror can take solar radiation which enters the HUD and focus this radiation onto the liquid crystal display (LCD) image source, or onto other parts that are made of plastic, causing physical damage to the LCD image source or other parts. As a result, commercially available HUDs often include a physically moveable shutter which is closed during the day so that little or no solar radiation can enter the HUD. However, while the shutter protects the HUD from damage caused by sunlight, it also prevents the HUD from being used during the day, because the HUD is prevented from projecting images onto the windshield when the shutter is closed.
A further consideration is that a suitable night vision infrared image, as presented to the driver, is customarily a low to medium resolution image having a horizontal field of view (FOV) of about 12xc2x0. There are commercially available LCD image sources which have been approved by vehicle manufacturers for use in automobiles, including an existing LCD which has a two-dimensional array of pixels configured as 320 pixels horizontally by 220 pixels vertically. This is a color device, in which each pixel actually includes a cluster of three pixel elements that respectively produce the three primary colors of red, green and blue, but for clarity here each such cluster of three pixel elements is referred to as a single pixel. This existing LCD is suitable for displaying a low to medium resolution night vision image with a 12xc2x0 horizontal FOV. However, it is not suitable for displaying, with a horizontal FOV of about 12xc2x0, a satisfactory image of alpha-numeric or graphical information that would be desirable for a day mode of operation.
In more detail, existing displays for night mode operation generally provide about 4.6 pixels per milliradium to the eye of the driver in the 12xc2x0 FOV. however, it is generally recognized that about 9 pixels per milliradium are needed to display alpha-numerical or graphical information of the type desired for a day mode of operation. Of course, it would be possible to provide a low resolution image source for use during the night mode and a separate high-resolution image source (such as a vacuum florescent display) for use during the day mode. However, the use of two separate sources is cost prohibitive as to the consumer vehicle market, and also presents the problem that it would be difficult or impossible to package both sources into the very limited physical space available for a HUD in a standard automobile dashboard.
Another possible approach would be to use a very high-resolution LCD image source for operation in both the day and night modes. However, no LCD manufacturer is currently known to offer such a device, and the cost of engineering and tooling needed to develop such a high-resolution LCD is very high.
From the foregoing, it may be appreciated that a need has arisen for a method and apparatus which facilitate the provision of a head-up display that is suitable for use in both a day mode and a night mode of operation, without any significant increase in size or cost in comparison to existing head-up displays that are operable only in a night mode. According to one aspect of the present invention, a method and apparatus are provided to address this need, and involve: using an image source to output radiation which represents a visible image; providing first and second reflective surfaces respectively having first and second reflection characteristics which are different; and directing the radiation from the image source to a viewing location according to a selected one of first and second modes, the first mode including reflection of radiation from the image source by the first reflective surface according to the first reflection characteristic, and the second mode including reflection of radiation from the image source by the second reflective surface according to the second reflection characteristic.